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From: Gerhard Hoffmann <dk4xp@arcor.de>
Newsgroups: sci.electronics.design
Subject: Re: noise question
Date: Tue, 16 Jul 2024 01:40:29 +0200
Message-ID: <v74c1d$kdm1$1@solani.org>
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Am 15.07.24 um 20:04 schrieb john larkin:
> On Mon, 15 Jul 2024 19:33:52 +0200, Jeroen Belleman
> <jeroen@nospam.please> wrote:
> 
>> On 7/15/24 18:09, john larkin wrote:
>>> On Mon, 15 Jul 2024 17:35:08 +0200, Jeroen Belleman
>>> <jeroen@nospam.please> wrote:
>>>
>>>> On 7/15/24 16:30, john larkin wrote:
>>>>> Does a negative 50-ohm resistor make as much noise as a regular 50 ohm
>>>>> resistor?
>>>>>
>>>>> I'd sorta guess the current noise to be the same, and maybe the
>>>>> open-circuit voltage noise is infinite.
>>>>>
>>>>> I could Spice that, at least the current noise, if Spice handles it
>>>>> right. LT Spice noise analysis is kind of weird.
>>>>>
>>>>
>>>> I just tried it: In LTspice the sign doesn't matter,
>>>> only the absolute value. Also, if you put a positive
>>>> resistor in series with negative one, the noise
>>>> voltages add RMS-wise, like you'd expect of independent
>>>> sources.
>>>
>>> Cool. Thanks.
>>>
>>>>
>>>> In real life, a negative resistor may have more or
>>>> less noise than an actual resistor, depending on the
>>>> low-noise design skills of the designer.
>>>>
>>>> I think you knew that...
>>>>
>>>> Jeroen Belleman
>>>
>>> Sure, I was considering an ideal neg resistor, without added noise
>>> from active parts.
>>>
>>> As a college project, I built a 2-terminal negative resistor and
>>> plugged the negative value into a bunch of equations (voltage
>>> dividers, RCs, LRCs, things like that) and demonstrated that they
>>> worked that way in real life. That was fun.
>>>
>>> What I was thinking lately was about making an LC oscillator with very
>>> low phase noise, namely low jitter in my world. The finite Q of the
>>> parallel LC is equivalent to a shunt resistor so I'd expect it to have
>>> the Johnson noise of that equivalent resistance. Then the active stuff
>>> must look like a negative resistor, which is noisy too.
>>
>> Yes, that's what I'd expect too.
>>
>>>
>>> LT Spice noise analysis is very limited. I have sometimes added some
>>> random-noise BV blocks in series with resistors and such, so I can do
>>> genuine nonlinear sims with noise. It's actually easier to breadboard.
>>>
>>
>> BTDT. What's with the nonlinear bit? LTspice noise analysis is
>> basically an AC analysis, no?

What irritates me is, that I need to edit .noise and .ac commands if I 
want to see noise and some gains; I want want both at the same time.
The way it's now is clumsy.

> LT Spice noise analysis is weird. You need one signal source, even if
> you don't use it. And you can only probe one node. It must be entirely
> linear.

At least, that seems to work.
< 
https://www.flickr.com/photos/137684711@N07/51974219045/in/album-72157662535945536/lightbox/ 
    >
> 
> I want to simulate jitter in a LC oscillator, and of course an
> oscillator always has some nonlinear amplitude limiting mechanism.
> 
If you want nonlinear noise, you need Keysight's Advanced Design System
or such. Be prepared to a 5 or 6 digit price tag, depending on options.
The keyword is harmonic balance simulator.

cheers, Gerhard